The overall objectives of the present proposal were to isolate the human populations of T, B, and Null cells in high yield with a high degree of purity and viability; to examine the isolated populations of cells and determine whether individually or in recombined pools, they mediate, in vitro, a wide variety of immunologic functions; to develop heteroantisera to these functionally unique populations of cells with the view of defining the heterogeneity which exists within these subsets; to examine the differentiative history of the hyman lymphoid cell; and develop an understanding of the regulatory events necessary for the differentiation of these specific effector populations in man.